Demands to increase storage capacity of storage devices know no bounds, since demands on storage, typically of image processing, have been increasingly made. Hard disc drives (HDDs) using magnetic recording have features of large capacity, non-volatility, and low cost and thereby hold a central position of storage devices. The market of such hard disc drives are now expanding, because they are more widely applied to fields of audiovisual apparatuses such as video recorders, and car navigation systems, in addition to personal computers. The market demands to minimize the size of memory. It is now predicted that memory capacity of 100 gigabits per square inches can be achieved with a magnetic material size of about 30 nm (300 angstroms) in 2004, and memory capacity of 1000 gigabits per square inches can be achieved with a magnetic material size of about 10 nm (100 angstroms) in 2010.
To allow magnetic materials for storage to have a finer size and a higher density (integration), the sensitivities of reproducing heads or magnetic sensors must be increased. The sensitivity is represented by magnetoresistive (MR) ratios. Downsizing of reproducing heads have been achieved by increasing MR ratio of sensing elements constituting the reproducing heads.
Regarding the MR ratio, giant magnetoresistive (GMR) devices with MR ratio of 4% have been developed and brought into practice since around 1994 (Patent Document 1). Those with MR ratio of about 10% are about to be released. At a storage density of 100 gigabits per square inches or more, however, the MR ratio of the GMR devices is still insufficient, and MR ratio of about ten percent to about several ten percent is required.
Tunnel magnetoresistive (TMR) devices having a MR ratio of 10% or more in development phase were achieved in 2000. The TMR devices could achieve a higher MR ratio that could not be achieved by conventional GMR devices (Non-patent Document 1).
Such TMR devices can be applied not only as magnetic sensors but also as magnetic memories. IBM announced a joint development project for launching 256 megabit-MRAM on the market in 2004. Accordingly, the importance of the TMR devices is increasing.                Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-11330        Non-patent Document 1: Ohashi et al., NEC “Low Resistance Tunnel Magnetoresistive Head”, “IEEE Transaction on Magnetics, Vol. 36, No. 5, pp. 2549-2553, 2000        Non-patent Document 2: M. Bowen et al., App. Phys. 82 (2003) 233        Non-patent Document 3: M. Kawasaki, Y. Tokura et al., Jpn. J. Appl. Phys. Vol. 42 (2003) L369-L372        